Spin Transitions DOI: 10.1002/anie.201205573 Cooperative Spin Transition in a Mononuclear Manganese(III) Complex** Paulo N. Martinho, Brendan Gildea, Michelle M. Harris, Tibebe Lemma, AnilD. Naik, Helge Müller-Bunz, TiaE. Keyes, Yann Garcia, and Grace G. Morgan* Spin-crossover (SCO) molecules are a class of compounds that show promise for application in memory devices. [1] A hysteretic transition pathway is essential in promoting a memory effect, and considerable efforts have been expended to develop systems with wide hysteresis loops close to room temperature. These efforts were led by Kahn et al., who published the first SCO Fe II complex with a RT hysteretic loop. [2] Many more examples have since appeared, [3] which are mostly polymeric Fe II complexes with triazole [4–6] or pyrazine ligands [4, 7] or mononuclear Fe II com- plexes with N-heterocyclic ligands, [8] but several hysteretic Fe III [9] and Co II [10] complexes are also known. In 2008, Weber et al. described a 70 K RT hysteresis for a mononuclear Fe II complex, demonstrating the potential of intermolecular interactions to promote effective cooperativity and sharp transitions. [11] Bousseksou, McGarvey, et al. have shown how pulsed light can be used to switch the spin state on the hysteresis loop of [Fe(pyrazine){Pt(CN) 4 }], underscoring the very real possibility of harnessing SCO into a data storage device. [12] In contrast to Fe and Co, there are few examples of SCO in Mn III . [13, 14] Mn III is a particularly interesting candidate for SCO as it has a pronounced Jahn–Teller (JT) effect in the high-spin (HS) state. The paucity of SCO data on this ion limits generalizations about how the JT distortion will affect coupling of the SCO to the lattice, but it may result in different profiles to those observed to date with Fe II and Fe III . Indeed the original [Mn(pyrol) 3 tren] SCO complex [13] has an unusual double transition profile, which prompted several experimental [15] and theoretical studies. [15a, 16] We have shown the importance of ligand flexibility in promoting Mn III SCO where population of d x 2 Ày 2 on switching to HS requires elasticity in the xy plane. [14a–d] This is readily achievable with L1 and its derivatives, which have an appropriately tuned N 4 O 2 ligand field and the necessary equatorial flexibility. To date, this system has promoted gradual SCO profiles [14a–d,f] but we now report the first example of a Mn III compound with an abrupt complete thermal spin transition and opening of an 8 K hysteresis window in [MnL1]PF 6 (1). The perchlorate salt [MnL1]ClO 4 has previously been reported as being HS. [17] Magnetic data of a crystalline sample of (1) were recorded in cooling and warming modes between 300–10 K (Figure 1). At room temperature, c m T = 2.79 cm 3 mol À1 K(S = 2) with g = 1.93, which persists on cooling to 132 K when it drops sharply to c m T = 1.25 cm 3 mol À1 K over a 4 K range with critical temperature T c fl = 130 K. There is a further decrease to c m T = 1.09 cm 3 mol À1 K, with g = 2.08 by 100 K, that is, full conversion to S = 1, followed by a drop below 25 K owing to spin–orbit coupling. On warming, a similarly abrupt transition is observed but with T c › = 138 K, resulting in a thermal magnetic hysteresis loop of 8 K centered at 134 K, which is much wider than for the [Mn(pyrol) 3 tren] complex with the same type of magnetometer (1.6 K loop). [15d] The crossover profile was sensitive to particle size, as revealed by temper- ature-cycling experiments on a second crystalline sample. These also exhibited an 8 K-wide hysteretic transition with identical c m T values in the HS and LS regimes and no fatigue over three cycles of heating/cooling (Supporting Information, Figure S2), but the transition was centered at a slightly lower temperature (closer to 132 K). An abrupt transition was also [*] Dr. P. N. Martinho, B. Gildea, M. M. Harris, Dr. H. Müller-Bunz, Dr. G. G. Morgan School of Chemistry & Chemical Biology, University College Dublin Belfield, Dublin 4 (Ireland) E-mail: grace.morgan@ucd.ie Dr. A. D. Naik, Prof. Dr. Y. Garcia Institute of Condensed Matter and Nanosciences, MOST-Inorganic Chemistry, UniversitØ Catholique de Louvain (Belgium) Dr. T. Lemma, Prof. T.E. Keyes School of Chemical Sciences, Dublin City University (Ireland) [**] We thank Prof. Maria JosØ Calhorda, Universidade de Lisboa, and Prof. Luis Vieros, Universidade TØcnica de Lisboa, for DFT calculations and Dr. Charles Harding, Open University (UK), for calculation of g values. Funding from the Irish Research Council for Science, Engineering and Technology (IRCSET), FNRS (FRFC 2.4508.08, 2.4537.12, IISN 4.4507.10), Fundażo para a CiÞncia e a Technologia, Enterprise Ireland, and the ERA-Chemistry programme is gratefully acknowledged. Significant funding for a SQUID magnetometer by the Irish Higher Education Authority is also gratefully acknowledged, as is generous support from Uni- versity College Dublin (to G.G.M. and B.G.). T.E.K. and T.L. acknowledge the Irish Government’s Programme for Research in Third Level Institutions, Cycle 4, Ireland’s EU Structural Funds Programmes 2007–2013 and Career Enhancement and Mobility Fellowship co-funded by Marie Curie Actions. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201205573. A ngewandte Chemi e 12597 Angew. Chem. Int. Ed. 2012, 51, 12597 –12601  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim